This invention relates to an improvement in the process for the electrochemical conversion of conjugated diene compounds into alkadienedioic acids.
Most particularly, this invention relates to the nature of the solvent utilized in the electrolysis reaction of conjugated dienes and carbon dioxide, and to processing improvements for which a certain class of solvents are found to be responsible. It is reported in the art (Fioshin et al, Chem. Abstr. 71 (1969)) that conjugated dienes, e.g., butadiene, can be converted into higher polyunsaturated dicarboxylic acids by the electrolysis of butadiene and oxalic acid in an alcohol solvent. It is now observed, however, that carrying out an electrolysis reaction in such a manner results in the formation of a polymeric layer at the anode surface, producing a strong anode polarization and substantially blocking the transfer of current to the electrolysis medium.
It is further disclosed in the art (R. V. Lindsey et al, J. Am. Chem. Soc. 81 (1959), 2073-2074) that butadiene can be converted into diethyl-3,7-decadiene-1,10-dioate by electrolysis in a methanol or methanol/water solvent and in the presence of a mixture of a half ester of a lower dicarboxylic acid and a half ester salt of the same dicarboxylic acid serving as a source of carbethoxy radicals. Apparently, a large variety of products are prepared by this method. Lindsey et al also report that electrolysis of methanol solutions of ethyl hydrogen maleate and either butadiene or isoprene formed films on the anode which greatly reduced the conductance of the electrolytic cell and the efficiency of the desired reactions.
The electrochemical conversion of conjugated dienes into alkenoic acids and alkenedioic acids, for instance, the conversion of butadiene into pentenoic and hexenedioic acids, is disclosed in U.S. Pat. Nos. 3,032,489 and 3,344,045. These patents, however, fail to describe the preparation of alkadienedioic acids. With specific regard to the use of anhydrous aprotic solvents according to the invention, the teachings of U.S. Pat. No. 3,032,489 disclose that a number of aprotic substances are suitable as solvents in the electrolysis mixture. However, the patent further teaches that the electrolysis solvent medium preferably also contains an amount of water, i.e., between 0.5 and 25 percent by weight based on solvent, which serves as a source of hydrogen ions. Water is said to be unnecessary if the solvent functions as a source of such ions, in which case the solvent would not, by definition, be described as aprotic.